A wind turbine generally includes two or more rotor blades secured to a hub that is rotatably coupled to a shaft. The shaft is mounted within a housing or nacelle, which may be positioned on top of a tower. The rotation of the bladed hub transforms wind energy into a rotational torque or force that drives one or more generators coupled to the shaft.
Wind gusts have been a formidable hurdle facing modem wind turbine designers. Sudden appreciable changes in wind velocity and/or direction can over-stress or damage components and result in premature failure of the wind turbine. Early efforts to mitigate the negative effects of wind gusts focused on mechanically changing the pitch angle of the rotor blades using a technique commonly known as auto-furling, in which a pitch drive system changes the pitch angle of the rotor blades, thereby changing the aerodynamic torque of the rotor.
While not very precise, auto-furling tended to reduce high failure rates, but not enough to satisfy design margins over the long-term. Subsequent efforts by electrical engineers focused on slip-enhanced electrical designs. This approach allowed the generator to slip much more than normal when the wind provided more power than the system was designed for and thereby created a little time for the pitch control system to respond to the new wind conditions. While slip-enhanced electrical designs proved superior over the mechanical auto-furling approach, but these designs proved inadequate as well. The changes in wind velocity and/or direction simply occur faster than control systems can adapt. For example, even with current high-speed data acquisition systems and signal conditioning units, a wait time or delay of 50 milliseconds is not an unreasonable expectation, and good sensor information could take significantly longer. Aside from the time lag in acquiring and conditioning useful signal data, the system must then respond in a timeframe that prevents damage.
Wind gusts vary widely. In general, higher steady state wind speeds have gusts with lower relative magnitudes. For example, wind gusts might double a steady state wind speed of 12 miles per hour (mph), peaking at 24 mph, but a 30 mph steady state wind speed might gust to only 50 mph. In addition to the magnitude of the gust, its duration is a very significant variable. Long duration wind gusts allow a control system to respond in time, and very short duration wind gusts are effectively attenuated by inertial effects. Unfortunately, wind gust durations between 3 and 6 seconds are very common, but not long enough for the control system to respond in time, and too long for the inertia of the system to simply absorb. The 3-6 second gust imposes one of the most difficult torque and power transient conditions designers must address and both mechanical solutions and electrical solutions have proved inadequate. Historically, this has forced a trade-off between a robust design and a reasonable cost. Engineering and quality assurance departments compromise with marketing and sales departments to mitigate cost increases for robust designs in exchange for “reasonable” failure rates in the field. Current failure rates of transmissions alone cost approximately $50,000 per year per megawatt. This cost is borne by the customers, investors, insurance companies, and manufacturers in various degrees. While insurance companies may have been the first to limit their exposure to transmission failure costs, investors and customers will soon follow.